Apple has just announced that iOS 9.3 will be available today as a free update to all. This update will be available at some point today via iTunes or through an over-the-air update.
The biggest feature of iOS 9.3 would have to be Night Shift mode, which changes the temperature of the display depending on the time of day. Out of the box it will be set to change with sunrise and sunset in your area, but you can customize it to specific times. You can also now password protect your Notes and this will work across iOS and OS X.